The present invention relates to a process for liquefying a stream of hydrocarbons such as natural gas, in particular in a process for producing liquefied natural gas and liquid CO2.
In typical natural gas liquefaction plants, refrigerants streams are used to produce the refrigeration at various levels of a main heat exchanger by vaporizing against the stream of hydrocarbons to be liquefied (typically natural gas).
It is desirable to liquefy natural gas for a certain number of reasons. By way of example, natural gas may be stored and transported over long distances more easily in the liquid state than in the gas form, since it occupies a much smaller volume for a given mass and does not need to be stored at a high pressure.
Typically, natural gas contains hydrocarbons and CO2 (0.5 mol % to 5 mol % approximately). In order to prevent the CO2 from freezing during the liquefaction of the natural gas, it is advisable to remove it. One means for removing the CO2 from the natural gas stream is for example amine scrubbing upstream of a liquefaction cycle.
Amine scrubbing separates the CO2 from the feed gas by scrubbing the natural gas stream with a solution of amines in an absorption column. The CO2-enriched amine solution is recovered at the bottom of this absorption column and is regenerated at low pressure in a distillation (or stripping) column. At the top of this distillation column, a CO2-rich acid gas is released. Thus the amine scrubbing treatment of the natural gas stream releases a CO2-concentrated “acid gas” stream, usually emitted directly into the atmosphere.
In natural gas liquefiers (50 000 tonnes per year to 10 million tonnes per year) the amount of CO2 emitted is sufficient (amount of CO2 emitted possibly ranging up to 200 tonnes per day) and it is possible to purify this CO2-rich “acid gas” to give food grade CO2.
Specifically, in the food field, in accordance with the current legislation, in order to be able to be sold, the CO2 produced must meet strict specifications in terms of quality and purity. Thus, for example, any trace of hydrocarbons or of sulphur derivatives must be eliminated (content typically less than 1 ppm by volume).
This purification is carried out by means of a dedicated CO2 purification unit requiring the installation of a dedicated refrigeration cycle (typically a refrigeration system operating with ammonia for example).
The operation of the “refrigeration unit” refrigeration cycle consists in providing the refrigeration necessary for the CO2 purification/liquefaction process.
Typically, a standard CO2 unit contains the following steps:                Step 1: Compression of the impure CO2 to a pressure between 15 and 50 bar abs.        Step 2: Purification of the CO2 for example by processes that use regenerative adsorbents, absorbents or catalyst to eliminate any presence of water, mercury, hydrocarbons and sulphur derivatives (non-exhaustive list of impurities).        Step 3: Distillation of the noncondensable gases in order to separate in particular oxygen and nitrogen from the CO2 produced.        
Thus, conventionally in a CO2 purification/liquefaction unit, it is necessary to provide refrigeration at three temperature levels:
1. Refrigeration at −20° C./−30° C. used for step 3 described in the paragraph above.
2. Refrigeration at 5° C. used for step 2.
3. Refrigeration at ambient temperature for cooling the impure CO2 in step 1.
The condenser of the distillation column used in step 3 represents around 50% of the total of the refrigeration requirements. This refrigeration may be provided via a dedicated refrigeration cycle (typically an ammonia or propane refrigeration cycle) optionally coupled with a water cooling system.
The system for producing frigories represents a high cost of the CO2 purification and liquefaction unit and adds complexity of implementation to the site for implemention of the process which represents a constraint.